The most sophisticated integrated circuits today have millions of components and require complex fabrication techniques. The techniques, beginning with the design of a complete mask set, generally require an extended time period to complete and are relatively expensive to implement. The expense is justified for integrated circuits that are produced either in high volumes or for high prices, but simplified techniques are desirable for circuits produced cheaply and in low volumes or which must be fabricated relatively quickly. Accordingly, approaches have been developed which do not require the complete customized mask set used with conventional fabrication techniques.
One approach uses a standard mask set for all integrated circuits and then customizes the circuits for particular applications after fabrication has been completed. This approach is commonly used for programmable logic arrays (PLA) and many read only memories (ROM). An essential step in the customization process is the selective opening or closing of an electrical circuit.
Several techniques have been developed to selectively open or close circuits, and the technique for closing an electrical circuit of interest for this application applies a voltage, in excess of a threshold voltage, across a structure and thereby changes the structure from a high resistance OFF state to a low resistance ON state. The structure is termed an antifuse in analogy with the better known fuse which changes from a low resistance ON state to a high resistance OFF state. Many techniques for fabricating antifuses have been developed. For example, a layer of amorphous silicon may be used between two conducting materials. When a voltage in excess of the threshold voltage is applied, the resistance of the amorphous silicon decreases dramatically. See, for example, U.S. Pat. Nos. 4,442,507 and 4,599,705 issued on Apr. 10, 1984 and July 8, 1986, respectively, and Cook, pp. 99-100, 1986 Bipolar Circuits and Technology Meeting for descriptions of representative antifuse techniques.
Within the circuit, the antifuses may perform different functions. For example, the antifuses may contain the stored information in a memory or they may be used to selectively connect different components within the circuit. It will be appreciated that in many applications the antifuse completes an electrical circuit through an interlevel dielectric. The details of the processes used to form an antifuse through an interlevel dielectric differ, but the process will pattern the dielectric to form windows which expose portions of the underlying material and then deposit a material, e.g., amorphous silicon, in the window and another conductive material on the amorphous silicon. A voltage is applied to program the device, i.e., to change its resistance.
This process produces adequate results for many applications, but it has been found that the programming voltage and the leakage current may vary significantly from device to device and that the ON state resistance varies slowly with time. In the process described above, it is difficult to control precisely the thickness of the amorphous silicon at the bottom of the window due to, e.g., poor step coverage. The window typically has an aspect ratio, i.e., ratio of window height to window width greater than 1:1. It is believed that this thickness variation is one of the processing parameters that leads to the undesirable variations in device characteristics mentioned.